Molybdenum (Mo) is the second element in Group VIB in the periodic table. It has an atomic number of 42, and atomic weight of 95.95, and valences of 2, 3, 4, 5, and 6. The average abundance of Mo in the earth’s crust is 1.2 ppm; in soils it is 2.5 ppm; in streams it is 1 μg/L, and in groundwaters it is <0.1 mg/L. Molybdenum occurs naturally as molybdenite (MoS2) and wulfenite (PbMoO4). It is used in alloys, ink pigments, catalysts, and lubricants.
The common aqueous species are HMoO4−, MoO42−, and organic complexes. It is considered an essential trace element for plants and animals. The United Nations Food and Agriculture Organization recommended maximum level for irrigation waters is 0.01 mg/L.
Use one of the flame atomic absorption spectrometric methods (Section 3111 D or E), the electrothermal atomic absorption spectrometric method (Section 3113 B), or one of the inductively coupled plasma methods (Section 3120 or 3125).